The invention relates to a high-pressure mercury vapour discharge lamp comprising an envelope which consists of a material capable of withstanding high temperatures and comprises electrodes of tungsten and a filling substantially consisting of mercury, rare gas and a halogen, free in the operating condition, for maintaining a tungsten transport cycle.
A superhigh-pressure mercury vapor discharge lamp of this kind known from DE-AS 14 89 417 has an elongate quartz glass envelope having a volume of 55 mm.sup.3. This envelope is filled with rare gas and 6.5 mg of mercury; this corresponds to a quantity of mercury of 0.12 mg/mm.sup.3. The mercury vapor pressure may be about 120 bar. The lamp has a power density of about 14.5 W/mm.sup.3. For lengthening the life, not only the wall of the envelope is cooled, for example by means of a flow of water, but also 5.10.sup.-4 to 5.10.sup.-2 g. atoms of at least one of the halogens per cubic millimetre are fed into the envelope.
Although such lamps at mercury vapor pressures of about 120 bar produce a high luminance, they yield essentially a typical mercury spectrum, which is superimposed on a continuous spectrum and contains a small red part.
GB PS 11 09 135 discloses a superhigh-pressure mercury vapor discharge lamp comprising a capillary tubular envelope of quartz glass, which is filled with mercury up to a quantity of 0.15 mg per mm.sup.3 of volume; this corresponds to a mercury vapor pressure of about 150 bar. In order to improve the color rendition, this lamp is moreover filled with at least one metal iodide. The high electrode load of these lamps results in that tungsten evaporates from the electrodes and is deposited on the wall of the envelope. This leads to a blackening of the envelope, as a result of which the latter is strongly heated, which may give rise to an explosion of the envelope especially at high mercury vapor pressures.